1.3.32Steque。一个以栈为目标的队列(或称为steque)，是一种支持push、pop、和enqueue操作的数据类型。为这种抽象数据类型定义一份API并给出一份基于链表的实现。

import java.util.Iterator;

public class Steque<Item> implements Iterable<Item>

{

    private int N;

    private Node first;

    private Node last;

   

    private class Node

    {

        Item item;

        Node next;

    }

   

    public boolean isEmpty()

    {return N==0;}

   

    public int size()

    {return N;}

   

    public void push(Item item)

    {

        Node oldfirst=first;

        first=new Node();

        first.item=item;

        first.next=oldfirst;

        if(isEmpty()) last=first;

        N++;

    }

   

    public Item pop()

    {

        Item item=first.item;

        first=first.next;

        N--;

        if(isEmpty()) last=null;

        return item;

    }

   

    public void enqueue(Item item)

    {

        Node oldlast=last;

        last=new Node();

        last.item=item;

        last.next=null;

        if(isEmpty())

            first=last;

        else

            oldlast.next=last;

        N++;

    }

   

    public Item dequeue()

    {

        Item item=first.item;

        first=first.next;

        N--;

        if(isEmpty()) last=null;

        return item;

   }

   

    public Iterator<Item> iterator()  {return new ListIterator();}

   

    private class ListIterator implements Iterator<Item>

    {

        private Node current=first;

        public boolean hasNext(){return current!=null;}

        public void remove(){}

        public Item next()

        {

            Item item=current.item;

            current=current.next;

            return item;

        }//end next

      }//end class ListIterator

    public static void main(String[] args)

    {

        Steque<String> q=new Steque<String>();

        while(!StdIn.isEmpty())

        {

            String item=StdIn.readString();

            q.enqueue(item);

         }//end while

        for(String item:q)

        StdOut.println(item+" ");

    }//end main

}//end class